1. Field of the Invention
The present invention relates to a method of producing a silicon carbide (SiC) semiconductor substrate and a semiconductor substrate obtained by the method, particularly, a method of producing a semiconductor substrate provided with a semiconductor device layer (epitaxial layer) with reduced number of defects, a silicon carbide semiconductor substrate obtained by this method and to a silicon carbide semiconductor using the silicon carbide semiconductor substrate.
2. Description of the Related Art
SiC single crystals are generally produced by a sublimation method. SiC single crystals produced in the method include defects such as dislocations (edge dislocation and screw dislocation (including a micropipe)). Specifically, if SiC substrate is formed by deposition of sublimated SiC on the seed crystal of SiC, this growth technique generate the defects easily, because this process is nonequilibrium of thermodynamic.
In manufacturing a power device or high-frequency device, an epitaxial layer (semiconductor device layer), the region where a device is formed, is grown so as to form a structure suitable to the device. However, if defects are present in the substrate, they are inherited by the epitaxial layer grown on the substrate to form almost the same number of defects in the epitaxial layer as well. It has been reported that when a device is formed on an epitaxial layer having defects, current leakage is increased and breakdown voltage is lowered.
Thus, in manufacturing the device, it is extremely important to reduce the defect.
To decrease the number of micropipes in the epitaxial layer where the device is formed, a method is proposed in which a SiC single crystal to be a substrate is subjected to high-temperature treatment so that the sublimated SiC precipitates in micropipes to plug them (see, for example, Japanese Patent Application Laid-Open No. 2002-179498). There has also been disclosed a method in which SiC is grown by a CVD (Chemical Vapor Deposition) method, subjected to heat treatment so as to plug the end of micropipes, then the plugged surface is exposed by thermal etching to be used as a seed crystal to grow a SiC single crystal (see, for example, Japanese Patent Application Laid-Open No. 2000-53498), and a method to grow SiC by a CVD method under a controlled film formation condition to develop micropipes toward the defects such as dislocations (see, Jpn. J. Appl/Phys. Vol. 41 (2002) L1300, L1137).
Although the micropipes are partly plugged by such methods they are converted into numerous dislocations, and therefore the above-mentioned problems are left unsolved.
That is, because the micropipe has a Burger's vector (“b”=|nc|; n represents an integer and c represents a displacement for one atom in the direction of c axis), which remains even though the micropipe is plugged by sublimation, it is divided into smaller dislocations of the Burger's vector. Thus, there can be generated dislocations of up to n per micropipe. Having an unpaired electron, the dislocation may lower the mobility of carriers by scattering them, or lower breakdown voltage by forming a low-resistance region which becomes a current leakage path.
In view of the above-mentioned situation, the inventors of the present invention have made earnest studies and found that the defects in an epitaxial layer can be suppressed by providing a buffer layer doped with germanium on a silicon carbide single crystal substrate.